Heat exchangers are commonly used in gas fired hot air furnaces in both residential and commercial settings. Heat exchangers are generally divided into two types. The first type includes tubular heat exchangers wherein a tube is formed into a serpentine configuration and hot combustion gases are allowed to propagate within the tube. The second type of heat exchangers more commonly used in compact designs are clam shell heat exchangers. Clam shell heat exchangers employ a pair of metal sheets or plates which are disposed in face to face relationship and are configured to provide a passageway for the flow of hot combustion gases. These type of heat exchangers are referred to as clam shell heat exchangers since they are formed of two separate mirror-imaged sheets which are joined together.
In typical use in a furnace, a series of heat exchangers are provided in which hot combustion gases pass through the heat exchangers transferring heat to the surfaces of the heat exchanger. Forced air passed externally over the heat exchanger is warmed and circulated into the room which is to be heated. To efficiently transfer the heat from the hot combustion gases to the heat exchangers, the heat exchangers are designed to cause a turbulent flow within the internal passageways. Turbulent flow causes the heated gases to interact with the walls of the heat exchangers so as to provide effective and efficient heat transfer.
Various techniques have been employed to provide turbulent flow in the heat exchanger passageways. U.S. Pat. No. 4,467,780 describes a clam shell heat exchanger having a series of dimples formed within the passageways of the heat exchanger. The dimples create obstacles within the gas flow stream thereby increasing the velocity of the combustion products and resulting in efficient heat transfer. U.S. Pat. No. 4,982,785 also shows a clam shell serpentine heat exchanger wherein a series of ribs and dimples are employed in the passageway to increase turbulence and facilitate heat transfer. U.S. Pat. No. 5,359,989 discloses a clam shell heat exchanger wherein each of the passageways in the heat exchanger is further divided into individual connected passageways. These passageways are of sequentially decreasing diameter so as to increase the velocity of the combustion gases passing therethrough. This is also designed to render the heat transfer more efficient. While each of the above-referenced patents attempt to maximize heat transfer between the combustion gases and the surface of the heat exchanger by increasing the velocity and the turbulent flow of the combustion gases within the heat exchanger passageway, further improved heat transfer efficiency in a compact clam shell heat exchanger is desirable.